The present invention relates generally to sensors for use with moving sheets or webs of material and, more particularly, to a non-contact sensor system for measuring or sensing properties or characteristics of a substantially continuous sheet or web of paper.
Systems for measuring characteristics or properties of moving sheets or webs of material are well known in the art. Typically, these systems employ first and second sensors in the form of sensing heads or shoes positioned on the opposite sides of a passing web. These sensors contain sensitive electronic, radiation or optical detection systems for measuring one or more characteristics of the passing web, such as thickness, opacity, moisture, gloss, smoothness, or other properties. Commonly, the moving web travels in a free gap between two scanning sensor heads and sheet properties are measured via an arrangement with sensing devices in upper and lower heads. In order to measure certain sheet properties, including but not restricted to thickness (also known as caliper), gloss or smoothness, a controlled and close proximity of the sensing heads to the sheet surface is advantageous in order to achieve acceptable accuracy. This can be partially accomplished by pass line control from sheet guide devices attached to the sensor heads, or preferably by using flexible mounts for the system that permit relative sensor movement in at least the vertical plane. An example of such a system and, in particular, one type of sensing head for contacting and measuring characteristics of a passing web of paper, is shown and described in commonly assigned U.S. Pat. No. 5,479,720 to Hellstrom et al., the disclosure of which is incorporated herein by reference.
In order to ensure that the selected characteristics or properties of the web are accurately measured, it often is desirable to position the opposed first and second sensors as close as possible to the web without contacting the web. Also, the first and second sensors must be in close alignment with one another in the web plane to ensure that any measurements taken correspond to substantially the same area of the passing web. This alignment requires careful manual adjustment of the sensing heads as well as costly precision scanning mechanisms, but can still never be fully attained. Simultaneously meeting both requirements is complicated by the fact that the pass line of the web relative to the sensors may rapidly change as a result of events occurring upstream or downstream of the system. Accordingly, not only must the mounting arrangement be capable of securely and reliably holding the sensors in a precisely controlled, spaced, and aligned relationship adjacent to the corresponding side of the web, but it must also be capable of rapidly responding to changes in the pass line. Also, contact or engagement between the sensors must be avoided to prevent the instrumentation held therein from damage, especially when the web of passing material is absent from the feed path.
In the ""720 patent, the sensors are designed to make actual physical contact with the passing web. This is possible due to specialized low-friction, wear-resistant contact surfaces formed of ceramic materials. A flexible mounting also ensures that the sensors are not only kept aligned in the web plane, but may also move as necessary in the vertical plane to ensure that the sensing heads can accommodate any rapid changes in the pass line of the web. Despite the advances offered by this solution, each sensor still directly contacts the web of passing material during sensing, which is not the most desirable for sensing or detecting properties of characteristics of certain web materials due to possible disruption or damage of the web at the location of contact.
One method to avoid web contact is to deploy a large free gap between upper and lower heads. This eliminates any sheet contact but typically reduces sensor accuracy since the sheet can flutter anywhere between heads and the sheet may not be flat, or parallel with the gap. Prior art suggests remedies with pass line control devices including rollers, air guides and vacuum plates to hold the moving sheet at a controlled position. This is difficult to accomplish on a fast moving or non-flat sheet and it solves only part of the problem. Examples of devices for sheet pass line control are disclosed in U.S. Pat. Nos. 4,877,485 (Carson); 4,449,398 (Williams); and, 5,654,799 (Chase). These methods have the common disadvantage of controlling the pass line to only one of either the upper or lower heads since the pass line cannot be controlled to both heads simultaneously due to variable head alignment.
In order to achieve non contacting thickness measurement in close proximity with the sheet, others have proposed supplying pressurized air to form a gas bearing between a single sensor or a pair of opposing sensors on one or both sides of a passing web. Usually, the air bearing sensors are supported by fixed or flexible mountings. These mountings create a measurement force against the process in order to balance the repelling force created by the air bearing(s) from moving the sensors to permit accurate measurements. However, in such a passive arrangement, the measurement force must be carefully controlled to ensure that the sensor(s) remain even approximately spaced at the desired distance from the web at all times, or a complex pneumatic or mechanical system is required. Furthermore, since the sheet may have curl, waves and draw wrinkles, it is not possible to apply a sensing force that is always at a normal to the sheet plane in such arrangements. Where two opposed sensors are provided, creating the desired spacing using air bearings with fully articulated mountings would require complex designs.
Non-contacting measurement of sheet thickness using a magnetic measurement system separated by air bearings on one or both sides of the process is known in prior art, for instance as disclosed in U.S. Pat. Nos. 5,243,849 (Williams); 4,528,507 (Williams et al.); 4,647,855 (Berglund); 5,865,059 (Alessandro); 4,292,838 (Larsen); and, 4,107,606 (Typpo). Although these designs eliminate sheet contact on one or both sides of the sheet, the accuracy of sensors using these methods has not been acceptable due to excessive web influence parameters including web flutter, waves and smoothness changes as well as measurement errors caused by head misalignment.
Other non-contacting thickness measurement methods have been suggested including distance measurement across a pair of large free gap sensing heads that measure location of the upper and lower paper surface relative to each sensor head augmented with gap measurement devices for measuring the head to head separation. Examples of methods for optical thickness measurement using this arrangement are disclosed in U.S. Pat. Nos. 5,210,593 (Kramer); 5,805,291 (Calvin, et al.); 5,355,083 (George, et al.); 4,358,960 (Porter); 6,281,679 (King) and, WO00/37885 (King, et al.). An example of ultrasonic thickness measurement using this arrangement is disclosed in U.S. Pat. No. 5,113,358 (Reber). The prior art involves sheet surface location sensing devices spaced by a certain large distance from the sheet, to maintain a safe separation for no contact with the process, in conjunction with magnetic gap measurement. The large separation distance presents a challenge to measure a large dimension accurately in order to estimate a sheet thickness value that is much smaller than this distance. Product quality requirements for fabrication of many paper products demand measurement errors no larger than one half micron (0.5xc3x9710xe2x88x926 meter) at any point across the web. This has to be fulfilled despite severe environmental conditions, a scanning device with certain mechanical errors between upper and lower heads, plus a process with variable pass line, curl and sheet surface conditions. Thus, in principle, these methods produce a non-contacting thickness measurement; but in practice, they never have achieved acceptable measurement accuracy in typical paper industry applications.
On-line measurement of optical surface properties, like gloss or smoothness, on one or both sides of paper webs can have measurement errors introduced by sheet flutter, non-flat sheets, sensor head deflections or vibrations. Examples of prior art gloss measurement are disclosed in U.S. Pat. Nos. 6,233,053 (Preston); 6,031,620 (Typpo); and 4,830,504 (Frohardt). An example of prior art on-line smoothness measurement is disclosed in U.S. Pat. No. 5,654,799 (Chase, et al.). The main unresolved problem in this prior art is the inability to simultaneously control a moving web relative to measurement heads on each side of the web, since each head is subject to deflection errors and sheet flutter. Another problem is introduced by the large free gap necessary for a safe non-contacting web passage. These problems makes it difficult to construct a physically small sensor with a narrow measurement area, since there is a general scaling rule that dictates the size of the optical system in relation to the maximum free distance to the web. Such constraints have limited the accuracy and practicality of on-line measurement of gloss and smoothness as well as additional sheet properties including formation, brightness, opacity, color, basis weight and moisture.
Accordingly, a need exists for a web material property sensing platform in which opposed first and second non-contact sensors remain aligned in the web plane and evenly spaced in close proximity to a web of passing material at all times, even when the pass line or sheet curl changes rapidly, and a method for achieving accurate measurement of thickness, optical and other sheet properties by means of integrating magnetic, optical or other sensing elements in this platform with a close proximity to, but not contacting, the process surface.
This need is met by the invention of the present application wherein a non-contact, sheet sensing system employs first and second sensors positioned on opposite sides of a passing web with the sensors being simultaneously repelled from the passing web and attracted to each other. The resultant net force keeps the sensors closely and evenly spaced from the passing web to ensure that reliable and precise sensing or detecting functions are provided. Since the sensors are repelled from the web, they react substantially instantaneously to any changes in the pass line or curling of the web as it moves along the feed path while the attractive force keeps any relative tilting or movement of the sensors in check. In one embodiment of the invention, an automatic actuator/retractor retracts the sensors to ensure that the sensors do not contact each other when the web is absent from the feed path, and may also be used to withdraw the sensors from adjacent the web as necessary. The sensing system results in a great improvement over prior art efforts to provide non-contact sensors for a passing sheet or web of material, especially in terms of operational reliability and accuracy.
Thus, the sensors of the present application enable non-contact sensing of surface properties on one or both sides of the web, as well as reflective/transmissive properties of the web by sensing elements located within the sensors in close and controlled proximity to the web surfaces. Web properties that can be measured by sensing elements in the sensors may include, but are not restricted to, caliper, surface smoothness, gloss, brightness, opacity and formation.
In accordance with a first aspect of the present invention, a non-contact system for sensing or measuring a property or characteristic of a sheet or web of material moving along a feed path comprises a first sensor positioned adjacent a first side of the feed path. The first sensor includes a passage for receiving pressurized gas and directing the gas toward the web when it is present in the feed path. A second sensor is positioned adjacent a second side of the feed path opposing the first sensor. The second sensor also includes a passage for receiving pressurized gas and directing the gas toward the web in the feed path. At least one magnet is mounted in each of the first and second sensors, with the magnet of the first sensor being aligned with and attracted to the magnet of the second sensor and forming a magnetic coupling. This magnetic coupling urges the sensors toward the corresponding side of the web and one another, with the gas directed toward the web from the first and second sensors forming gas bearings that simultaneously urge the sensors away from the web. As a result, a net force generated by the magnets and the gas bearings keeps the sensors substantially evenly spaced from the web passing along the feed path, thereby ensuring accurate sensing and measuring of the desired properties or characteristics of the web.
To permit relative movement of the sensors in the vertical plane, a first flexible mount is provided for the first sensor and a second flexible mount is provided for the second sensor. This relative movement capability ensures that the first and second sensors may remain substantially parallel to the passing web of material at all times, even during changes in the pass line and the process itself. At least one of the first and second flexible mounts may also permit movement of the associated sensor in the horizontal plane. However, as should be appreciated, the associated sensor is prevented from moving any significant distance in this plane relative to the other sensor when the first and second sensors are magnetically coupled to one another.
The magnet of the first sensor may comprise a first plurality of magnets that align with and correspond to a second plurality of magnets comprising the magnet in the second sensor, with north and south poles facing each other to create an attraction force.
The sensing system may also include an actuator associated with at least one of the first and second sensors. In operation, each actuator enables or urges the associated sensor to move toward the feed path and the opposite sensor. Preferably, first and second actuators are associated with the first and second sensors, respectively, with the first and second actuators together enabling or urging the first and second sensors to move toward one another.
In one possible embodiment, each actuator comprises a pneumatic cylinder including a plunger having a first head for engaging the corresponding sensor. When each of the cylinders is pressurized, the corresponding plunger moves the sensor associated therewith toward the feed path and the opposite sensor. Each plunger further includes a second head disposed in the pneumatic cylinder for engaging a spring held therein. Upon de-pressurizing the pneumatic cylinders, the springs bias the corresponding plungers away from the web and thereby retract the corresponding sensors. Thus, each sensor xe2x80x9cfloatsxe2x80x9d on the first head of the corresponding plunger upon actuation as a result of the net force created by the combination of the attractive force supplied by the magnetic coupling and the repelling force created by the gas bearings, as well as the flexible mountings that permit the sensors to move up and down in the vertical plane and to tilt. Preferably, each pneumatic cylinder is in communication with and activated by the pressurized gas also used to form the gas bearings. Accordingly, the first and second sensors are automatically retracted upon a loss of pressure in the pressurized gas.
In accordance with a second aspect of the present invention, a non-contact system for sensing or measuring a property or characteristic of a web of material moving along a feed path is provided. The system comprises first and second sensors positioned on opposite sides of the feed path. Each of the first and second sensors includes at least one magnet, an inlet for receiving a pressurized gas, and at least one outlet for directing pressurized gas toward the feed path to create first and second gas bearings for the web of material when present. The at least one magnet of the first sensor is aligned with and attracted to the at least one magnet of the second sensor by magnetic coupling, which in turn aligns and urges the first and second sensors toward one another and the web. However, the gas directed from each of the first and second sensors to form the first and second gas bearings simultaneously urges the sensors away from the web and one another so that the net force keeps the sensors closely and substantially evenly spaced from the web moving along the feed path.
In one embodiment, the sensor system also comprises a first retractor for engaging and moving a first sensor away from a second sensor and the feed path defined between the first and second sensors, and a second retractor for engaging and moving the second sensor away from the first sensor and the feed path. Each of the first and second retractors comprises a cylinder including a pressure-activated plunger having a first head for engaging the corresponding sensor and a second head disposed in the cylinder for engaging a spring held therein. Thus, upon relieving the pressure in each cylinder, the spring biases the second head of the plunger such that the first head engages and moves the corresponding sensor away from the feed path. Preferably, the cylinders of the first and second retractors comprise pneumatic cylinders, but the use of other equivalent arrangements is of course possible.
In accordance with a third aspect of the present invention, first and second sensors are provided for use in an overall system for measuring or sensing a property or characteristic of a web of material moving along a feed path. Each sensor comprises a sensor head including at least one magnet, an inlet for receiving pressurized gas, and at least one outlet for issuing the pressurized gas towards a passing web of material to form a gas bearing. The first and second opposed sensors are simultaneously attracted and aligned by the magnets and repelled by the gas bearings to keep the first and second opposed sensors evenly spaced from the passing web.
In accordance with a fourth aspect of the present invention, a method of sensing or measuring a property or characteristic of a web of material moving along a feed path is provided. The method comprises positioning first and second opposed sensors adjacent to and on opposite sides of the feed path. Each of the sensors includes a magnet. Together, these magnets form a magnetic coupling that serves to align and draw the sensors toward one another and the web of material passing through the feed path. The method also includes the step of pressurizing passages in the sensors to form a fluid bearing on either side of the web so that net magnetic and bearing forces keep the sensors closely and substantially evenly spaced from the passing web. The method may also include the step of retracting the sensors when the web of material is not to be sensed.
In accordance with a fifth aspect of the present invention, a method of sensing or measuring a property or characteristic of a web of material moving along a feed path comprises positioning a first sensor on one side of a web of material in the feed path and positioning a second sensor on a second side of the web of material. The first and second sensors are drawn toward one another and the web of material with a magnetizing force, and repelled from one another and the web with a repelling force. The magnetizing force and repelling force are controlled to maintain the first and second sensors closely and evenly spaced from the passing web.
In one embodiment, repelling the first and second sensors from one another and the web comprises forming gas bearings in the first and second sensors and applying pressurized gas to the gas bearings. Also, drawing the first and second sensors toward one another and the web of material comprises mounting at least one magnet on the first sensor and mounting at least one magnet on the second sensor, wherein the magnet on the first sensor and the magnet on the second sensor are polarized to attract one another. Preferably, the first and second sensors have a substantially flat surface facing the feed path, and the at least one magnet of the first sensor is recessed below the substantially flat surface of the first sensor, while the at least one magnet of the second sensor is recessed below the substantially flat surface of the second sensor.